In a brushed DC motor, the brushes make mechanical contact with a set of electrical contacts provided on a commutator secured to an armature, forming an electrical circuit between the DC electrical source and coil windings on the armature. As the armature rotates on an axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches that operate in sequence such that electrical power flows through the armature coil that is closest to the stator, which houses stationary magnets creating forces relative to the coil windings that cause rotation.
A brushless DC motor makes use of control circuitry to operate switches that replace the combination of brushes and electrical contacts on the commutator. While the control circuitry can add to the expense of the brushless DC motor, the elimination of the brushes and commutator reduces maintenance, as there is no wear on an associated brush, and prevents arcing in the motor that can occur as the commutator moves past the brushes. In some examples, a plurality of Hall effect sensors and magnets are disposed on the rotor and armature, with the outputs of the Hall effect sensors used to control current switching.
An example of a brushless DC motor appears in FIGS. 1A–1B. The motor 10, shown in cross section, includes an armature 12 and a rotor 14, with magnets 16, 17 disposed on the rotor 14. Hall effect sensing elements 18 are disposed on the armature 12 to sense the location of the magnets 16, 17. Control circuitry 20 includes Hall interrupt detection block 22 that is coupled to the Hall effect sensors 18 and generates an interrupt or other signal whenever one of the Hall effect sensors 18 transitions, indicating the rotation of the magnets 16, 17. When triggered, the detection block 22 interacts with commutator state circuitry 24 to control changing of the state of output switches 26. Using the output switches 26, the control circuitry 20 can couple energy from line power 28, which typically (though not necessarily) passes through a step-down transformer 30 to the motor 10.
FIG. 1B shows a different cross section of the motor 10, with magnets 16, 17 on the rotor 14, and three coil windings 32, 34, 36 on the armature 12. Usually, during operation, two of the coil windings 32, 34 will be activated while a third coil winding 36 will be grounded. FIG. 2 illustrates the timing of operation. In FIG. 2, a trio of armature coil windings are indicated as A, B, and C, with the state indicated by a “+” (driven by a voltage V), “−” (grounded) or “0” (open circuit). Alternatively, a dual power supply approach would have “+” be a signal of a first polarity, “−” be a signal of a second polarity, and “0” indicate that the winding is grounded. At each Hall effect sensor interrupt 38, the commutator state is changed, and the current flow through two of the coil windings changes. Operation is further illustrated in FIG. 3, which shows operation during a run state. As shown at 40, the control circuitry waits for a Hall effect sensor interrupt or trigger, detects a transition of the output for one of the Hall effect sensors, as shown at 42, and changes commutator state, as shown at 44. The control circuitry then returns to 40. Other tasks may also be performed, but the basic steps are shown. Refinement of this process to improve efficiency and output actuation is desirable.